The conventional ring laser gyroscope is based on two counter-rotating optical resonator modes. These modes split in frequency when the gyro rotates about its sensitive axes. The frequence split, which is proportional to the rotation rate and the area enclosed by the optical mode is measured by the use of optical heterodyne detection of the two frequencies. Ring laser gyroscopes show much promise for eliminating the spinning-mass gyroscopes with their many moving parts, high cost, complexity, and in some cases inaccuracy and unreliability. The accuracy of prior art ring laser gyroscopes has been limited by the well known lock-in phenomena where the two frequencies tend to be locked together and do not split for low rotation rates. Many well known techniques have been devised to partially remove or eliminate the lock-in limit. Two of these techniques utilize two ring lasers located very close to each other and operated so that the light wave travels in only one direction in each ring laser. In one of these, Sepp, et al., U.S. Pat. No. 4,035,081, this control is accomplished by incorporating in each cavity an active optical element such as a magneto-optical crystal to limit the direction of travel. However, Sepp et al require externally applied magnetic fields and controls which produce waste heat and, thus, requires additional active techniques to compensate for frequency changes produced by the temperature gradients. Using the techniques of Sepp et al allows the direction of travel of the waves in the two ring lasers to be reversed by reversing the fields on the active elements in the cavities, which allows one to attempt to compensate for nonrotational induced frequency changes--such as effects due to manufacturing tolerances and stress induced changes in path length. The stress may come from mechanical or temperature changes or from a combination of these. It would be more desirable to accomplish this reversal of the direction of travel in a passive way.
The other application of two ring lasers, Roberts et al, U.S. Pat. No. 4,521,110 issued June 4, 1985 is issued to the inventors of the subject invention, Thomas G. Roberts and Thomas E. Honeycutt. Roberts et al utilize a completely passive technique, referred to as negative feedback, to cause light to travel in only one direction in each cavity. This eliminates the need for external power supplies and controls, internally applied fields, active optical elements, and reduces the need for compensation of temperature variation induced effects. However, in both of these cases it is desirable to construct the two optical cavities as nearly identical as possible and to control the environment so that all changes in frequency are the results of rotations only. Roberts et al also disclose in detail background attempts to satisfactorily circumvent lock-in, degenerate modes of conventional ring lasers and associated loss problems, and phase matching problems encountered in prior art devices.